In many industrial applications valves and other fluid handling components are subjected to varying temperature conditions. In some situations, for example, valves and other fluid components are thermally cycled through extreme temperatures ranging from elevated temperatures of several hundred degrees Fahrenheit to very low temperatures of 40 degrees Fahrenheit below zero or lower. Among other significant problems resulting from such extreme changes in temperature is the maintenance of fluid seals at the joints of fluid handling components. Valves and other fluid containment components frequently are formed of metals. Sealing materials used at the joints of such components, however, frequently are formed on non-metallic material. Depending upon the material from which the sealing material is formed, the coefficients of thermal expansion and contraction for the sealing material frequently is substantially different than the thermal coefficient of expansion and contraction of the fluid handling or containment components. As a consequence of these differences in the coefficients of thermal expansion, the sealing relationship between the sealing material and the components often is lost or compromised. Many conventional sealing materials, such as graphite, have coefficients of thermal expansion that are less than the coefficients of the material of the components they are used to seal.
Many times, the fluids being handled in applications involving conditions of extreme thermal cycling are highly corrosive. When such is the case, many of the conventional elastomer materials used for sealing, such as rubber, are unacceptable, since the corrosive fluids will often attack the sealing material. In such situations, seals often are formed of fluorinated hydrocarbon polymers, such as polytetrafluoroethylene. Many of these fluorinated hydrocarbon polymers are inert to virtually all chemical media and are suitable for use with a wide range of corrosive fluids. Fluorinated hydrocarbon polymers have a high coefficient of thermal expansion relative to the metal materials used for most valves, fluid containment devices and related components.
Regardless of whether the coefficients of the sealing material are greater than or less than the materials of the components the sealing material is used to seal, any substantial difference in the thermal expansion or contraction rates generally creates problems with the seal integrity when the components are subjected to thermal cycling. These complications are, of course, compounded considerably when the thermal cycling occurs between temperature extremes.